Based on three periods of crustal deformation monitoring data from 2018 to 2020 of Wudongde reservoir on the lower reaches of Jinsha River in China, we study the crustal deformation characteristics and mechanisms before and after reservoir impoundment. The results show that: 1) Reservoir impoundment has a significant impact on crustal deformation in the impoundment area, but has a weak impact on crustal deformation in the non-impoundment area. 2) Reservoir impoundment causes a crustal relative uplift in the impoundment area, which generally exceeds 10 mm and the maximum value exceeds 30 mm. 3) Reservoir impoundment induces a tensile strain towards both banks in the impoundment area, which generally exceeds 5 mm and the maximum value exceeds 10 mm. 4) Reservoir impoundment causes a noticeable increase in gravity values in the impoundment area, which generally exceeds 200 μGal and reaches 900 μGal in some local areas. 5) The increase of water mass and the rise of pore water pressure in the rock layers on both banks after reservoir impoundment may be the main reason for the crustal deformation of Wudongde reservoir. 6) The findings of this study differ from the crustal deformation results caused by the impoundment of the Three Gorges reservoir, which has an important reference significance for understanding the characteristics and deformation mechanisms of crustal deformation caused by reservoir impoundment.